This application claims priority from European Patent Application Nos. 02080031.4, filed Dec. 2, 2002 and 03076401.3, filed May 9, 2003, which are herein incorporated by reference in their entirety.
1. Field of the Invention
The present invention generally relates to a lithographic apparatus and more particularly to a lithographic apparatus having an adjustable position dependent intensity distribution.
2. Description of Related Art
The term “patterning means” or “patterning structure” as here employed should be broadly interpreted as referring to structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such a device is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors and unaddressed mirrors will reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix-addressing can, for example, be performed using suitable electronic means. In both of the situations described above, the patterning means can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and
A programmable liquid-crystal display (LCD) panel. An example of such a device is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as set forth above.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper or step-and-repeat apparatus. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic apparatus as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a device manufacturing process using a lithographic projection apparatus, a pattern (e.g., in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g., an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing,” Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens,” however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example, whereby any of these types of projection system may either be suitable for conventional imaging or be suitable for imaging in the presence of an immersion fluid. The radiation system may also include components operating according to any of these design types for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle,” “wafer” or “die” in this text should be considered as being replaced by the more general terms “mask,” “substrate” and “target portion,” respectively.
In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range 5–20 nm), as well as particle beams, such as ion beams or electron beams.
The radiation system has to ensure the realization of a desired intensity distribution as a function of position across the beam and as a function of angle of incidence of rays in the beam. The intensity distribution may either be defined at mask (reticle) level or at substrate (wafer) level. The desired position dependence, excluding mask generated patterns, generally should be uniform with constant intensity as a function of position on the substrate, and the desired angle dependence has to peak at certain angles. The desired angle dependence may depend on the nature of the pattern on the mask. Therefore it has to be possible to change properties of the illumination of the mask, so that the appropriate desired angular dependence can be realized. Very complex illumination systems are to ensure realization of the desired intensity distributions.
European Patent Application No. 1304850.9 (with corresponding U.S. application Ser. No. 10/157,033 assigned to the same assignee as the present application and having common inventors), which is incorporated herein by way of reference, describes examples of illumination systems. One illumination system has a laser source, followed by an optical element such as a Diffractive Optical Element (DOE), an axicon and a zoom lens. Following the DOE, axicon, and zoom lens, the beam passes through an optical integrator rod. The rod evenly smears out the intensity distribution as a function of position, conserving most of the angle dependence. This means that at the exit side of the rod, the spatial intensity distribution over the complete cross-section of the rod is almost perfectly uniform.
The DOE, axicon and zoom lens are used to shape the angle dependent intensity distribution of the projection beam at the substrate (“angle” in this context may refer to the angle relative to the main direction of the beam, as well as to an angle rotated around that main direction). As an alternative to at least the DOE a matrix of individually, electronically orientable micro-mirrors or micro-lenses has been proposed. By controlling the fractions of mirrors that reflect the beam in various directions very detailed control of the angle dependent intensity distribution is made possible. It has also been proposed to perform the function of the axicon by a DOE.
Such a highly complex illumination system should be accurately configured to ensure the desired uniformity and intensity distribution as a function of angle at the substrate. Small deviations in the illumination system may affect the intensity distribution. Also erratic factors, such as contamination may be of importance.
Generally, a correct intensity distribution is ensured by a set-up procedure, which involves choosing the components needed for illuminating a particular mask and adjustment of the parameters of these components prior to exposing the substrate, to ensure that the desired intensity distribution as a function of position and angle will be realized. The parameters involve for example the distance between the elements of the axicon and/or the orientation of the mirrors in the matrix of mirrors.
It has proven possible to measure the position dependence of the intensity distribution at the substrate. For this purpose a detector may be included near the substrate, or near the mask, where there is usually sufficient space for such a detector.
Measurement of the angular intensity distribution is less straightforward. In principle the angular distribution can be measured directly with position dependent intensity detection in or near a pupil plane, that is, inside the optical system. Alternatively, a pinhole can be placed at mask level while measuring the defocused image of the pinhole in the form of a defocused spatial intensity distribution at substrate level, which corresponds to the angular intensity distribution at mask level. In both scenarios however, the normal imaging process is interrupted. The relation between a pupil plane and the plane of the mask is that the intensity distribution as a function of position in the pupil plane determines the intensity distribution as a function of angle at the mask. Vice versa the intensity distribution as a function of angle in the pupil plane determines the intensity distribution as a function of position at the mask (although the latter relation is altered in a lithographic apparatus by passing the projection beam through an internally reflecting rod). This relation arises because between the pupil plane and the mask an optical structure is included with an effective focal distance so that the pupil plane is at the effective focal distance from the optical structure.
Measurement of position dependent intensity at a pupil plane currently involves an interruption of normal operation of the apparatus, because it may involve putting a detector into the pupil plane. In the set-up for normal operation many components of the illumination system generally crowd the space in the vicinity of the pupil plane, for example because they are needed to control the illumination pattern at the pupil plane. Obviously, when the measurement of position dependent intensity at a pupil plane is alternatively performed using a detector at an object or image plane, normal operation of the apparatus is also interrupted.
After checking the intensity distribution at the pupil plane measured with the detector and adjusting the parameters of the optical components that affect the intensity distribution in the pupil plane, the apparatus is restored to normal operating order so that the beam can reach the substrate. The optical components that affect intensity distribution in the pupil plane are subsequently left as set-up to retain the required angle dependence.
This procedure has the disadvantage that it increases the time before the apparatus can be used after a change of mask. Moreover, it excludes dynamic control over the angle dependent intensity distribution of the radiation beam.